The invention relates to an electrochemical detector for detecting electroactive substances, i.e., substances which are either oxidizable or reducible. Such detectors belong to the most sensitive and most specific detectors presently available and are of particular advantage in liquid chromatography where they are used to detect the liquid eluting from the separation column.
An electrochemical detector for detecting electroactive substances is known, for example, from EP-A- 140286. This known electrochemical detector comprises an electrochemical cell into which the liquid to be analyzed is introduced and in which three electrodes are arranged: a working electrode, a counter electrode (also denoted as auxiliary electrode), and a reference electrode. The electrochemical process is made to occur at the working electrode and the reference electrode compensates for any change in the conductivity of the mobile phase transporting the substances to be analyzed. The potential near the counter electrode is held at a fixed value by a control circuit commonly denoted as "potentiostat". The potential near the counterelectrode is sensed by the reference electrode which is connected to the potentiostat. When a substance to be analyzed arrives at the surface of the working electrode, a current is developed which is converted by an electrometer to a voltage output which can then be processed by further circuitry.
In the prior art according to EP-A- 140286, the liquid to be analyzed impinges perpendicularly onto the working electrode in the form of a liquid jet; this type of detector design is called "wall jet design". Other detector designs employing a three-electrode configuration are also possible, for example the "thin-layer design" wherein the liquid flows past the working electrode. In both designs, reference electrodes are used which comprise a redox couple, such as the commonly used Ag/Ag+Cl- redox couple.
There are several potential sources of error in an electrochemical detector which may lead to inaccurate measuring results. Such errors typically lead to a shift of the so-called "half wave potential". The term "half wave potential" will be shortly explained: When the potential of the working electrode versus the reference electrode (where both of them are referred to ground) is increased and the current measured at the working electrode is plotted as a function of the potential, a characteristic curve results which has a flat portion at low potentials, then a portion where the current increases steeply and then again a substantially flat plateau. The point on this curve where the current value is half of the value of that of the plateau corresponds to a certain potential which is called the half wave potential. In order to have reproducible detection conditions, it is desirable that the half wave potential for a specific substance to be detected remains constant.
It may occur, however, that the half wave potential for a specific substance changes. For example, it is frequently the case that the working electrode is passivated in the course of the electrochemical detection process due to contamination. As a result of this, the curve of current versus potential becomes flattened and the corresponding half wave potential moves to a higher value, thus impairing the reproducibility of the measurement. One way to overcome the problems with passivation of the working electrode is to operate the detector in a pulse mode as described in the above-mentioned prior art, whereby one portion of the pulse cycle serves to clean the working electrode.
Other error sources in an electrochemical detector, particularly when used in connection with a liquid chromatograph, may be due to problems in the liquid delivery like flow ripple or bad degassing or gas bubbles in the electrochemical cell itself. The mentioned error sources may all contribute to an overall error in the detector; the prior art approach to cope with such errors is to eliminate the individual sources of error, for example to provide means for cleaning the working electrode (e.g., by using pulse mode), or to ensure that the flow conditions are as uniform as possible. The potential of the reference electrode may change due to contamination of the electrode or by changes in the ion concentration in the environment of the electrode, and thus also be a source of error. Apparently, however, the reference electrode has not been much considered in the prior art as an error source.